Tuned radio frequency coupling device



- g i i 3 i 6 Nov. 10, 1931. w. VAN B. ROBERTS 1,331,640

TUNED RADIO FREQUENCY COUPLING DEVIQE Filed Feb. 13,1928

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INVENTOR WALT VAN B. ROBERTS BY nzj/ QM TORNEY Patented Nov. 10, 1931 1 UNITED" STATES PAT NT. OFFICE WALTER van 3. ROBERTS, or PRINCETON, vnew JEnsnY, ASSIGNOR T0 RADIO CORPORA- "Tron OF AMERICA, A CORPORATION OF DELAWA TUNED RADIO FREQUENCY COUPLING DEVICE Applieation filed February 13, 1928. Serial NO. 253,831. 7

One of the diflioulties .encountered in mod ern radio practice is the fact that unless numerous adjustments areprovided, unlform results will not be obtained when recelving 6 signals over the usual frequency range. It

has been found that this is due mostly to the fact that the ordinary coupling means now in use have changing characteristics with changing of frequencies of current. It is,

1 therefore, one of the objects of this invention means.

Another object of the invention is obtaining of uniformselectivity throughout the broadcasting range. I

Still another object of the invention is to obtain uniform amplification throughout the broadcast range.

Other objects of the invention will be apparent in'the description of the device when read in connection with the drawings, in which;

Fig. 1 shows a preferred embodiment 'of the invention;

Fig. 2 illustrates a modification of them vention slightly differing in detail from Fig. 1;

Fig. 3 is similar to Fig. 1 except that the arrangement thereindisclosed allows forJthe.

grounding of the rotor of the variable con denser; r

Fig. shows' a'circuit incorporatingthe i11- vention broken up into its constituent parts; Fig. 5ashows the coupling circuit of Fig. 1 detached and;

Fig. 5b shows'curves for certain conditions existing in the circuit of Fig. 5a. 1

Briefly this invention deals with the design of a constant coupling circuit for vacuum tubes arranged in cascade relation. 'In priorv circuits, known inthe art, when producing approximately constant couplingfor a certainrange of frequencies. circuits have been designed in whichthe coupling between the tubes is the sum of an inductive and a capacitive coupling. The purpose of this dual coupling effect is that'upon a change infrequency-the lossin the coupling value of the capacitive coupling would be made up by the increase in inductive coupling, or vice versa,

sothat in rea'litythe result obtained would be, the algebraic sum of'the coupling effectsof" the two couplings, which by the proper choice of constants and electrical values of the 00111-1 ponentparts: could be made approximately constant for a certain band of predetermined frequencies.

that the'total 5 coupling effect is not a simple I to:; The present inventionidifi'ers therefrom in sumvof two simple -couplings but is a quan- I tity which mathematical analysis shows to be determined by the ratio of two other quan-' tities (neither of them couplingsin their.

own right) each of which is a function of frequency, and. which ratio is kept constant by choosing. circuit elements such that as one quantity increases withfrequency, the other alsoincreases in approximate proportion.

In a word the-oldsystem utilizes the constancy of a sum of two quantities onegoing up as the other goes down, while the present invention utilizes theconstancy obt'ainablein a'rajtio of two quantities which vary propor tionately. i 1

In the drawings, Fig. 1, two thermionic tubes 1 and 20f a cascadecirc'uit are-shown coupled-together by circuit 3, consisting of the inductances Li an'dL and the capacity C all ofwhich. are preferably units of fixed value, Anextremity-of each inductance is connected to the filament circuit and-the. op; posite extremities are connected to the opposite terminals of'the capacity C. The plate battery B may be connected to energize the: plate inany suitable manner for example in series with the coil L as shown. Coupling circuit. 3 is tuned to a frequency outside the rings, inthis case, lower than any frequency to be received by-properly selecting the values of units C, L and L. A series resonant circuit consisting of condenser 5 and inductance 6 is connected in parallel with inductance L Either or both of the elements 5, 6 may be made adjustable for tuning.

Fig.2 shows a different method of coupling in which 3 as before represents the coupling circuit composed of the fixed condensers (3 and C and the fixed inductance L. This modification, it will be noted, replaces the inductances of the coupling circuit of Fig. 1 by capacities and the capacity by an inductance. In Fig. 2 because of the arrangement a direct current path must be provided in the plate filament circuit which path is shown by choke coil 4; and battery B. In this circuit the coupling device is tuned by meansof C L and C to a frequency above any to be re ceived. Mutual inductance between the various coils in Figs. 1 and 2 is not necessary, but may be present without detriment if not excessive.

In the two circuit arrangements just discussed it will be noted that the rotor of the variable condenser 5 is not grounded. In case this is desired the apparatus may be arranged as shown in Fig. 3 which is similar to Fig. 1, except that the tuning inductance 6 in the variable condenser 5 have been transposed.

The inductance 6 has been shown variable as it would be preferable to do at least part of the tuning by varying the inductance, or otherwise the tuning range might not be sufficient and the voltage amplification might become relatively too large at the high frequency end of the scale. However, if necessary the tuning may be done entirely by either variable inductances or variable'condensers.

The component parts and further details of the circuit are shown for purposes of further explanation in Fig. 4: in which a circuit according to the invention is broken up into its fundamental constituent parts. In this figure the plate circuit of the tube 1 is energized in themanner shown in Fig. 2 and a blocking condenser 7 of large capacity is utilized to prevent short circuit through inductance L The input circuit of tube 2 is provided with a grid leak 8 and grid condenser 9 for enabling the tube to act as a detector in a well-known manner. The input vacuum tube 1 acts as if it were an alternating current generator having definite internal resistance. Next comes, a so-called transducer or coupling circuit 3, made up of inductances L and L and capacity C. The

term transducer is usedto designate any network of impedances which receives energy in one pair of terminals and delivers it usually at a different voltage out of another pair of terminals. The output terminals of the transducer are connected to the series combination inductance 6 and capacity 5. This series combination is utilizedfor the purpose of producing high voltage by passage of the current through large reactances and to select or tune by insuring that there is only one frequency at which this resonance voltage occurs. The last fundamental constituent is the input circuit of the next vacuum tube 2 which may be connected across any two points through Z and Z and the other component through Z will be say, 2' and the part through Z be 71 From well-known formulae The part going through Z Z will Let us designate by the word transducance the voltage appearing between the output terminals per ampere flowing into the input, the output terminals being supposedly unconnected to anything.

Therefore, from above g Z1Z2 Transducance i Z2 Z3 Plotting separately the values of the numerator and the value of the denominator 1n the above equation for a range of dlfierent frequencies the two curves "N and D shown in Figs. 5-6 will be'obtained, remembering always, of course, that for purposes of the curves certain values of L and L haveibeen chosen and that they remain constant throughout the curves. For other values of the constants, the values obtained from the curve would have to be multiplied by certain constants.

By means of Thevenins theorem, it can be proven that the uniformity of selectivity and amplification depends very approximately upon the transducance. To illustrate, suppose that the desired result is to have the same current flowing through the tuning elements at'all frequencies and further suppose that the resistance of the tuning ele ments does not vary with frequency. Then, the so-called tra-nsducance should remain constant in order to receive the desired retasters sult. If other results are desired, the transducance should vary with frequencies in a definite fashion.

Now in many cases, it would be desirable to have this transducance remain approximately constant. If a transducer is used such as shown in Figs. 1 or 2, the expression for the transducance takes the form of a ratio such as shown above by Equation and that by suitable choice of constants the numerator and denominator can be made to vary almost proportionately over the desired frequency range such as 50 to 150 kcs. so that the transducance remains substantially constant over the range. Such a condition would approximately exist for the frequencies shown be-- tween the points a and b of Fig. 56. It will be seen from that portion of the two curves that the values increase almost proportionately which means that the transducance will remain approximately equal for that range.

It is not intended to limit the invention to the particular construction and arrangement of parts shown, since many modifications may be made therein without departing from the invention and the appended claims should be construed to cover all such modifications.

I claim:

1. In combination a thermionic valve having input and output circuits said output circuit comprising a choke coil, a current source in series therewith and a capacity across said choke coil and said source, a second thermionic valve having input'and output circuits, said input circuit comprising a pair of condensers in series and an inductance in parallel therewith, and means for connecting one of said last named condensers to said first mentioned condenser comprising a circuit including an inductance. v

2. In an amplifier system for amplifying electrical impulses of radio frequency, the frequency of which may vary within specified limits, a source of radio frequency energy, a space discharge device having input and output circuits, a transducer for coupling said source and said space discharge device and maintaining a constant coupling relationship therebetween irrespective of whichever frequency within said limits is being transferred therethrough, said transducer having an input side and an output side said output side being connected to a series combination of inductance and capacitance at least one of which is adjustable for tuning said series combination to any frequency within said specified limits, the input of said space discharge device being connected across two points of said series combination having a difference of potential between them, said transducer comprising two reactances of like sign and one reactance of opposite sign connected in series, said reactances of like sign and said reactance of unlike sign beingtuned in combination to'a'frequency outside said specified limits, said output slde of, the transducer being formed across two pointsof one of said first mentioned likereactances, and said inputside being formed across two points of the other'reactances of like sign. 3 a g A system for couplinga source ofa'lternating current to'the input of a space dis chargedevice, adapted to maintain a constant coupling therebetween irrespective of the frequency transferred within a certain frequency band to be received, which comprises three reactancestwo of like sign and one ofunlike sign forming a series circuit, the total reactance of said circuit vanishing at a frequency outside said band of frequencies to be received, means for coupling the source of alternating current to one of the two reactances of like sign, means for coupling the other reactance of similar sign to a series combination of unlike reactances one of which is adjustable for selection purposes and means for coupling one of the last named reactances to the input electrodes of the space discharge device. 7

4. A system for coupling a source of alternating current to the input of a space discharge device, adapted to maintain constant coupling irrespective of the frequency transferred therethrough within a frequency band to be received which comprises a pair of terminals for input energy an inductive reactance connected in shunt, a capacitive reactance in series, another inductive reactance in shunt and a series pair of reactances of opposite signs, one thereof being adjustable,

in shunt with said second inductive reactance the input electrodes of said space discharge. device being tapped across one of said series pair of unlike reactances, said first three named reactances being chosen to have the sum of their reactances vanish at a frequency below the range of frequencies to be received.

5. The combination of elements as recited in claim4 in which the magnitudes of each of the first named three reactances are such that the impedance measured across the third mentioned reactance when said three reactances are connected to the source of current, is small compared to the impedance of the series pair of reactances of opposite signs.

6. A system for coupling a source of alternating current to the input of a space discharge device adapted to maintain constant coupling irrespective of the frequency within a frequency band to be received transferred therethrough, which comprises a pair of input terminals, a capacitive reactance connected in shunt, an inductive reactance in series, another capacitive reactance in shunt and a series pair of reactances of opposite signs, one thereof being adjustable, in shunt with said second capacitive reactance, the input electrodes of said space discharge device being tapped across one of said series pair of reactances, said first three named reactances being so proportioned that the sum of their reactances vanish at a frequency abovethe range of frequencies to be received.

7 The combination as recited in claim 6 in which the magnitudes of each of the first three named reactances are such that the impedance measured across the mentioned reactance when said three reactances are connected to the source of current is small compared to the impedance of either of the series pair of reactances.

WALTER VAN B. ROBERTS.

DISCLAI M'ER 1,831,640.Wclter Van B. Roberts, Princeton, N. J. TUNED RADIO FRE UENCY COUPLING DEVICE. Patent dated November 10, 1931. Disclaimer filed February 3, 1933, by the assignee, Radio Corporation of America.

Therefore, enters the following disclaimer:

In claim 3, no claim is made to the combination recited except Where the resonant frequency of the combination of the first-named reactance 0f unlike sign and that reactance of like sign which is coupled to the series combination of unlike reactances is higher than the lowest frequency of the range to be received.

[Ofiicial Gazette February 21, 1983.] 

